A wear phenomenon (hereinafter referred to as fretting) called a fretting wear occurs on various mechanical parts such as parts aimed at restraining a relative motion of a fit-in portion of a shaft, a bolted portion, a riveted portion, a tapered coupler, and the like; and parts such as a rolling bearing, a sliding bearing, a ball bush, a spline bearing, a flexible shaft coupling, a universal joint, a constant velocity joint, a laminated leaf spring, a coil spring, an electric contact point, a valve, a valve seat, a wire rope, and the like which have a slight reciprocating motion. In transporting cars a long distance by using a trailer, a freight train or the like, the fretting may occur on a rolling surface of the bearing owing to slight vibrations which occur during transport. In the case where a bearing is used in a low-temperature environment such as a cold region, lubricating oil cannot be sufficiently supplied to the rolling surface thereof. As a result, damage such as the fretting is liable to occur.
There is a possibility that the fretting occurs on motors to be used in environments in which parts of the motors have slight reciprocating motions and vibrations. In motors for industrial machines such as a servo motor, a stepping motor, and the like, there is a fear that the fretting occurs on the rolling surfaces of the bearings due to vibrations of parts thereof which occur when the motors are in operation and slight reciprocating motions of parts thereof. There is a fear that the fretting occurs on the rolling surfaces of the bearings of motors for electric equipments for use in cars and on the rolling surfaces of the bearings of motors for driving electric cars and hybrid cars owing to slight vibrations caused by an engine power and the like and owing to slight vibrations caused by circumstances of road surfaces while cars are traveling. In the case where lubricating oil is not sufficiently supplied to the rolling surfaces of the bearings in low-temperature environments, the fretting is liable to occur.
In conformity to a tendency that a motor is miniaturized in recent years, the miniaturization of a bearing for supporting the rotor of the motor is being advanced. Therefore there is tendency that a high contact surface pressure is applied to members constructing the bearing for the motor. Further there is a large increase in the rotational speed of the motor. Thus when the operation of the motor is started and stopped, there is tendency that the bearing for the motor has a high acceleration and deceleration in the rotation of the motor. Owing to an increase in a surface pressure at a portion between rolling elements and a bearing ring and an increase in sliding at the above-described portion caused by a sudden acceleration and a sudden deceleration in the rotation of the bearing for the motor, an oil film is liable to break (defective lubrication) at the above-described portion. Thereby metal contact occurs and thus the rolling element and the bearing ring are liable to wear, which increases the degree of fear of the occurrence of the fretting.
A rolling bearing and particularly a large double-row roller bearing as shown in FIG. 5 are frequently used as a bearing supporting a main shaft of a large wind power generator. A rotor blade 32 is mounted on a rotor shaft (main shaft) 33. The rotor shaft 33 is rotated by a wind power received thereby. The rotation of the rotor shaft 33 is increased by a speed-up gear 36 to rotate a power generator 37 and generate an electric power. While the electric power is being generated by the received wind power, an axial load (axial load applied to bearing) and a radial load (radial load applied to bearing) generated by a wind power applied to the rotor blade 32 are applied to the rotor shaft 33 supporting the rotor blade 32. A blade bearing for use in a blade pitch pivotal seat and a yaw bearing for use in a yaw pivotal seat are used for the wind power generator. To receive wind efficiently, the blade bearing is mounted on the base of a blade to rotatably support the blade so that the angle of the blade can be adjusted in dependence on the strength of the wind. The yaw bearing pivotally supports the yaw of a nacelle to adjust the direction of a main shaft according to the direction of wind.
Because the main shaft-supporting bearing, the blade bearing, and the yaw bearing are operated in environments where the direction and power of wind always fluctuate, the main shaft-supporting bearing, the blade bearing, and the yaw bearing are repeatingly oscillated in opposite directions in dependence on the direction of the wind and slowly and quickly by the strength of the wind. As a result, the fretting may occur on the rolling surface of the bearing. The main shaft-supporting bearing irregularly repeats start, acceleration, deceleration, stop with the main shaft-supporting bearing being always subjected to a load fluctuation. In addition, because vibrations of the blade and a gear box are applied to the main shaft-supporting bearing. Thus there is a high possibility that the fretting occurs on the main shaft-supporting bearing. In the wind power generator which is installed on the top of a mountain and in a very cold district and is thus subjected to very low temperatures, grease hardens and thus lubricating oil cannot be sufficiently supplied to the rolling surface of the bearing. Therefore there is a fear that the fretting has a large damage.
Various methods for preventing the occurrence of the fretting have been proposed. In one of these proposed methods, an appropriate lubricant is selected to prevent the occurrence of the fretting. In this method, at least one substance selected from among oxidized paraffin, diphenylhydrogen phosphite, and hexamethylphosphoric triamide is added to the urea-based thickener (see patent document 1).
In a hub bearing for rotatably supporting wheels of a car, carbon steel for machine structural use such as S53C which has a high forgeability and is inexpensive is used in a second generation hub bearing (GEN 2) and a third-generation hub bearing (GEN 3) provided with a flange on the outer ring thereof. In the hub bearing composed of the carbon steel for machine structural use, by subjecting the raceway part to high frequency heat treatment, the rolling fatigue strength of the bearing part is secured. But the carbon steel for machine structural use contains a small amount of the alloy component and thus has a weak surface strength. Therefore the carbon steel for machine structural use is inferior to bearing steel in a resistance to peeling of grease arising from the surface of the bearing part. As a measure for solving the above-described problem of the carbon steel for machine structural use, grease containing zinc dithiocarbamate added to its base oil as its essential component is known (see patent document 2).
As grease for preventing the fretting from occurring in the hub bearing or the like in a wide temperature range from low to high temperatures by using base oil comparatively inexpensive, the grease containing the predetermined base oil, the poly(meta)acrylate, zinc dithiophosphate, and the phosphate is proposed (see patent document 3).